Energy & Fuels 1994,8, 851-855
851
A Model Study of the Thermal Decomposition of Cumene Hydroperoxide and Fuel Instability Reactions George W. Mushrush,*p+lfErna J. Beal,? Robert E. Pellenbarg,t Robert N. Hazlett,s Harold R. Eaton,t and Dennis R. Hardyt Navy Technology Center for Safety and Survivability, Naval Research Laboratory, Code 6181, Washington, D. C. 203754342) Chemistry Department, George Mason University, Fairfax, Virginia 22030, and Hughes Associates, University Boulevard West, Wheaton, Maryland 20902 Received February 1, 1994. Revised Manuscript Received March 29, 1994"
Reactions that lead to fuel instability can be closely linked to the presence of active oxygen species such as hydroperoxides. An increasing body of evidence links oxidation reactions and the increasing polarity caused by incorporation of heteroatoms into the sediment precursors that form insoluble macromolecular compounds. The active oxygen compounds present in fuels are alkyl and aromatic hydroperoxides. Cumene hydroperoxide represents a logical choice for an active oxygen compound that could be present in a middle distillate fuel. This paper reports on the reactions of cumene hydroperoxide in benzene solvent for a reaction temperature range of 130-170 "C for a 30-min time period. The complete slate of products is presented along with a suggested mechanism to explain the observed products and the implications for fuel instability reactions.
termination
Introduction Hydroperoxide concentration has been found to be a major factor in fuel instability.' Two distinctly different types of fuel instability reactions are of continuing concern to middle distillate fuel consumers. Both types of instability processes are defined in terms of the quantity of filterable sediment and gum formed. Degradation is observed in fuels under long-term,low-temperature storage conditions (storage instability) as well as short-term, hightemperature consumer conditions (thermal-oxidative instability),,A Even though the two instability processes would appear to be different, they both depend on an active oxygen species, Le., hydroperoxides, to initiate the instability reaction s e q u e n ~ e . ~ - ~ ~ ~ Hydrocarbon autoxidation is well understood and involves the following sequential steps.4 initiation R-H
+ In -.R' + In-H
(a)
-
(b)
propagation
R' + 0, RO,'
+ R-H
-
RO,'
R' + ROOH
(C)
* To whom correspondence should be addressed at NRL. + Naval Research Laboratory. t George Mason University. 1 Hughes Associates.
*Abstract published in Advance ACS Abstracts, May 1, 1994. (1)Watkins, J. W.; Mushrush, G. W.; Hazlett, R. N.; Bed, E. J. Energy Fuels 1989, 3 (2), 231. (2) Goetzinger, J. W.; Thompson, C. J.; Brinkman, D. W. US. Dept. of Energy Report No. DOE/BETC/IC-83/3,1983. (3) Taylor, W. F. Ind. Eng. Chem. Prod. Res. Dev. 1974,13, 133. (4) Scott, G. Atmospheric Oxidation and Antioxidants; Elsevier: Amsterdam; 1965; Chapter 3.
2R0,'
-
alcohol + ketone + 0,
RO,'
-
+ R' 2R'
ROOR
R-R
(d) (e) (f)
The initiation step, reaction a, affords an alkyl free radical. This step can be surface catalyzed. The propagation steps b and c carry the chain to the relatively stable hydroperoxide product. Step c is usually rate controlling, although at very low (ca. 1 ppm) oxygen concentration step b can be rate controlling. Termination reactions are also oxygen dependent with step d predominating at high oxygen concentration and steps e and f at low oxygen concentrations. Although slight oxidative degradation is observed in nonoxidizing atmospheres, the presence of hydroperoxides will greatly accelerate degradation as well as significantly lower the temperature at which fuel degradation occurs. The primary autoxidants present in fuels are molecular oxygen and hydroperoxide species? Degradation reactions usually are interpreted in terms of classical free radical mechanisms. Much of what has been reported about hydroperoxides and their reactions shows little resemblance to fuel systems.'-13 (5)Hazlett, R. N.; Schreifels, J. A.; Stalick, W. M.; Morris, R. E.; Mushrush, G.W. Energy Fuels 1991,5, 269. (6) Hazlett, R. N. Free Radical Reactions Related to Fuels Research. In Frontiers of Free Radical Chemistry; Pryor, W., Ed.; Academic Press: New York, 1980; p 195. (7) Coordinating Research Council. Determination of the Hydroperoxide Potential of Jet Fuels; Report No. 559; Coordinating Research Council: Atlanta, GA, April 1988. (8)Kharasch, M. S.; Fono, A.; Nudenberg, W. J.Org. Chem. 1951,16, 113. (9) Hiatt, R. R. Twenty Years of Peroxide Chemistry. In Frontiers of Free RadicaZ Chemistry; Pryor, W., Ed.; Academic Press: New York, 1980; pp 226-236.
This article not subject to U.S.Copyright. Published 1994 by the American Chemical Society
852 Energy & Fuels, Vol. 8,No. 4, 1994
This paper reports on the thermally initiated decomposition of a model system employing cumene hydroperoxide in both a shale-derived middle distillate fuel and benzene solvent at various temperatures. Cumene hydroperoxide represents a reasonable choice for a naturally occurring aromatic hydroperoxide present in middle distillate fuels. The benzene solvent precludes the reaction of the hydroperoxide with the solvent. Thus, the slate of products delineates the reactions of the hydroperoxide itself. The temperature range chosen for the model study, 130-170 "C, represents a heat load that turbine fuel could be reasonably subjected to under user conditions. The temperature/ time matrix for storage instability studies was 80 OC/14 days. This matrix accurately predicts fuel instability for a time period of up to two years of storage under ambient conditions. The results indicated that although radical oxidation kinetics are complex, it was possible to explain the distribution of both the major and minor products in terms of a few competing reactions. Furthermore, we have developed reaction conditions and an analytical method of high reproducibility which may be applicable to the study of other hydroperoxide oxidative processes. Experimental Section Reagents. Cumene hydroperoxide (CHP)was obtained from ICN Pharmaceuticals. It was purified to greater than 99.9% purity." Benzene was obtained from Aldrich (Gold Label). It was refluxed and distilled from calcium hydride. Caution Note. Anhydrous hydroperoxides can decompose violently, especially in the presence of trace quantities of metal ions. All reactionsthat involve a matrix includingperoxide species should be heated with care. We have had a few reaction tubes explode for unexplained reasons during the heating process. Shale Fuel. The fuel for the present instability study was a diesel fuel marine (DFM) refined from Paraho crude shale oil by SOHIO. This fuel has been well characterized.16 It contained the antioxidant 2,4-dimethyl-6-tert-butylphenol(AO-30) at a concentration of 24 mg/L of fuel. Method. For the modelstudy in benzene solvent,the reactions were carried out in sealed borosilicateglass tubes. CHP, typically 9 X lo-' mol, was weighed into 0.6 mL of benzene solvent. The Pyrex reaction tubes were 6 in. long with V4-in. 0.d. and flamesealed at one end and fitted at the other with a stainless steel valve via a Swagelok (Teflon ferrules) fitting. The tube was attached to a vacuum system, cooled to 77 K, and subjected to several freeze-pump-thaw cycles. The tube was then subsequently flame-sealed below the valve. The ullage volume (0.30 mL) was kept constant for all runs. The deaerated samples were warmed to room temperature and immersed in a Cole-Parmer fluidizedsand bathandstressedat 130-17OoC. Thetemperature was controlled by a Leeds and Northrup Electromax I11 temperature controller. The total pressure for each run was calculated to be 5.1 atm at 130 OC and slightly higher at the other temperatures. Samples were thermally stressed for a 30-min time period. After the reaction period, the sealed tubes were quenched to 77 K and opened. (10) Howard, J. A. Free-Radical Reaction Mechaniime Involving Peroxidesin Solution.In The Chemistry offinctionol Groups,Peroxides; Patai, S., Ed.; Wiley: New York, 1983; Chapter 8. (11)Wurster, C. F.; Durham L. J.; Mosher, H. S. J. Am. Chem. SOC.
19S7, 80, 327. (12) Howard, J. A.; Ingold, K. U. Can J. Chem. 1968,46, 2666. (13) Fodor, G. E.; Naegeli, D. W. Development of a test Method to Determine Potential Peroxide Content in Turbine Fuels. In Conference Proceedings, 2nd Internotional Conference on Long Term Storage Stabilities of Liquid Fuels; SouthwestResearch Institute San Antonio, TX, 1986; Vol. 2, pp 632-645.
Mushrush et al. A search of the literature gives a few examples of catalytic behavior with glass systems.14-17 However, when a glass reaction tube was partially fiied with crushed Pyrex, thus dramatically increaeingthe glass surfacearea,the results for the 30-min reaction time were not substantially altered. Instrumental Methods. The samples were analyzed by several instrumental techniques. The GC/MS unit consisted of a Hewlett-Packard Model 5710 GC, a Hewlett-Packard Model 5982A mass spectrometer, and a Ribermag SADR GC/MS data system. An all-glass GC inlet system was used in conjunction with a 50-m x 0.31-mm4.d. wall coated SP-2100 open tubular fused silica capillary column. Operational parameters included the following: sample size, 2-3-pL split at 601; column flow, 1.1-1.2 mL/min at a head pressure of 10.5 psi; detector gain, 9 X low; injection port, 250 "C; temperature program, initial temperature of 70 OC for 2 min with a programmed temperature ramp of 4 OC/min to a fiial temperature of 220 OC with a 16-min hold. All chromatograms were recorded and integrated on a Hewlett-Packard Model 3390A reporting integrator. The samples were analyzed by three gas chromatography techniques. Peak identification waa based on retention time matching with standards and mass spectrometry. In the fiist, a Varian Model 3700 GC with a fid and equipped with a 50-m X 0.21-mm4.d. wall-coated, OV-101, open tubular fused silica capillary column gave the necessary resolution to distinctly separate the individual components. A carrier gas flow of 1mL/ min was combined with an inlet split ratio of 6 0 1 and a temperature program with an initial hold at 50 OC for 8 min, a ramp of 4 OC/min, to a fiial temperature of 260 OC. In a second technique, the gases formed were separated from any residual liquid with a 6-ft X '/gin. stainless steel alumina column (42/60 mesh) and resolved by a 6-ft X '/gin. stainless steel column packed with 5 X molecular sieves. Column temperature was maintained at 100 OC. The reaction tube was conneded directly to the GC by a l/lrin. stainless steel tubing, evacuated, and then pressurized from the reaction tube. An inline pressure gauge measured the pressure before analysis by the helium ionization detector. A third technique was used for CO and COSand as a check for low molecular weight gases formed during the reaction. The gases were analyzed using a Perki-Elmer Model Sigma 2 gas chromatograph equipped with a 6-ft 5A Molecular Sieve column (CO and CIA)or a 4-ft Porapak/S column (Cod. For thie analysis, the column waa operated at 55 OC. An external standard was used for calibration. A pressure gauge measured the pressure in the sample loop at the time of analysis. A material balance was assessed for each compound. The principal peaks of the chromatogramaccountedfor approximately 97 % of the original compounds. The very small peaks accounted for another 1-2 % of trace producta. These trace products were not quantitated and consisted primarily of partially oxidized or other unidentified compounds. The product distribution was repeatable to 3-5% for each component. Infrared spectra were recorded on a Perkin-Elmer Model 681 spectrophotometer equipped with a Model 3600 Data Station. Solution proton NMR spectra were obtained with a Varian EM390 90-MHz instrument. Elemental analyses were accomplished with a Perkin-Elmer Model 240 elemental analyzer, with a CarloErba Model 1106 instrument, or at a commercial laboratory. Storage Test Technique. The accelerated storage stability test method used has been described.16J8The temperaturehime matrix used was 80 "C/14 days. This test regimen accurately predicts storage instability resulta for up to two years. Samples (14) Muehh,G. W.; Hazlett, R. N.J.Org. Chem. 1986,50(13),2387. (16) Beal, E. J.; Cooney, J. V.;Hazlett, R. N.; Morris, R. E.; M u e h h ,
G. W.; Beaver,B. D. Mechaniim ofsyncrude/SynfuelDegradation. Final Report, U.S.Departmentof Energy,Report No.DOE/BC/l0626-16,2987. (16) Kirk, A. D.; Knox, J. H. Trans. Faraday Soc. 1960,56,1296. (17) Howard, J. A.; Ingold, K. U. Con. J. Chem. 1967,43,785. (18) Wadik, N. J.; Robineon, E. T. Commercial Scale Refining of Paraho Crude Shale Oil into Military Specification Fuels; ACS Symposium Series; American Chemical Society: Washington, DC, 1981; Vol. 163, p 223.
Decomposition of Cumene Hydroperoxide
Energy & Fuels, Vol. 8, No. 4, 1994 863
were run in replicate. After the stress period,the test flasks were allowed to cool to room temperature before being filtered under slight vacuum through a double layer of Gelman glass fiber filter pads. Flask contentswere rinsed with n-heptanewith additional sediment being collected on the filter pads. The sediment was rinsed with n-heptane to remove absorbed fuel. The flasks and fiiter holders were heated to 120O C for 12h, allowedto equilibrate to room temperature, and weighed several times on an analytical balance. Appropriate blank flask/filter holder corrections were applied. The original fuel and the fuel fiitrates from the stressed samples were analyzed for hydroperoxide concentration by a standard iodometric titration procedure, ASTM D3703-85.1e Duplicate titrations were conducted for each sample of filtered fuel.
Results and Discussion
Scheme 1
-
thermal
initiation
R(CH,),CO'
+ 'OH
(8)
propagation
-
@-scission
R(CH,),CO' CHP + R(CH,),CO*
RCOCH,
-
+ 'CH,
hydrogen abstraction
(h)
+
R(CH,),COH
R(CH,),COO' (i) CHP + 'CH,
CHP + 'OH
-
hydrogen
R(CH,),COO'
+ CH4
R(CH,),COO'
+ H,O
2R(CH,),CO'
+ 0,
abstraction
hydrogen abstraction
2R(CH,),COO' termination 2R(CH,),CO'
-
-
-
acidic
CHP conditions R(CH,),CO+
[R(CH,),COl,
-
+ OH-
(P)
reanaagem ent
R(CH3)2CO+ RO(CH,),C+
The thermal decomposition of aryl hydroperoxides unlike that of peroxides is complex. There are many literature results that are difficult to interpret for a complicated matrix such as a middle distillate Most of the observed inconsistencies can be attributed to trace amounts of metallic ions, mineral acids, bases, and other polar species that would have a drastic effect on hydroperoxide reactions. The solvent and CHP used in this study were rigorously purified to preclude their presence.14 Two decomposition reaction schemes can be proposed. Scheme 1is proposed for the major products while Scheme 2 represents the minor products.8J0 Scheme 1is depicted
CHP, R(CH,), COOH
Scheme 2
RO(CH,),C+
(9)
-
+ R(CH,),COOH ROH + CH,C(O)CH, + R(CH,),COO'
(r)
in steps g-m, while Scheme 2 is represented by steps n-r.
A detailed free radical decomposition mechanism for a hydroperoxide is dependent on specificreaction conditions since free radical reactivity is sensitive to structure, solvent, temperature, type of impurity present, and stereoelectronic effects. At temperatures of 100 "C or less, CHP was stable for 24 h or more in purified refluxing hydrocarbon solvents, i.e., hexane or benzene. The results in Table 1 show that the observed major products can be explained by eqs g-m in Scheme 1. The values expressed in the table are in terms of mole percent. All of the values were based on the starting amount of CHP. The values expressed are thus greater than 100% since some of the products could result from several steps in the mechanism. The overall decomposition reaction of cumene hydroperoxide is not first order, but it is closer to first than to any other order.8*20Thus, we will consider the reaction to be first order in the following discussion. The major products from the thermal decomposition of CHP in benzene solvent included acetophenone, cumenol, and dicumene peroxide. Acetophenone, 16.9 mol % at 130 "C increasing to 40.8 mol % at 170 "C and cumenol, 27.8 mol % increasing to 46.2 mol 7%,were derived from the same reaction, the formation of the cumyloxy radical, reaction g. A subsequent @-scission,step h, would give rise to acetophenone, while hydrogen abstraction, steps j and k, would yield the cumenol. That the cumenol was always significantly higher in yield than acetophenone indicated that, in benzene solvent, the cumyloxy radical was abstracting a hydrogen from the starting CHP itself and that steps h and i are almost equal in probability. No evidence on solvent participation was noted in the reaction products. These results compare favorably with those of other studies employing alkyl-substituted hydroperoxides.21 The major difference observed was that an alkyl substituted hydroperoxide, tert-butyl hydroperoxide, was found to form very stable long-lived peroxy radicals with a subsequent increase in the hydrogen abstraction process.21*22 (19) ASTM. Standard Test Method for Peroxide Number of Aviation Turbine Fuels. In Annual Book of Standards; ASTM Philadelphia, PA, 1986; Part 0.05.05, ASTM D3703-85. (20) Denisov, E. T. Liquid Phase Reaction Rate Constants; IFI/ Plenum: New York, 1974. (21) Muahrush, G. W.; Hazlett, R. N.; Eaton, H. G. Ind. Eng. Chem. Prod. Res. Deo. 1985,24,290. (22) Howard, J. A.; Adamic, K.; Ingold, K. U. Can. J. Chem. 1969,47, 3793.
854 Energy & Fuels, Vol. 8, No.4, 1994
Mushrush et al.
Table 1. Mole Percent Yield of Products at Selected Temperatures for the Thermal Decomposition of Cumene Hydroperoxide in Benzene for a 30-min Reaction Time conversion (mol %) product
13OOC
14OOC
15OOC
160°C
170°C
acetophenone cumenol dicumene peroxide gaseous products methane oxygen minor products acetone ethane a-methylphenyloxirane phenol
16.9 27.8 2.5
24.7 30.1 2.7
29.1 35.2 3.1
37.0 43.7 3.9
40.8 46.2 4.1
24.2 4.1
28.7 4.7
33.9 5.6
41.0 5.9
44.3 6.7
0.1 0.2
0.3 0.2
0.5 0.2 0.1
0.7 0.3 0.1
0.6 0.3 0.1
0.2
0.5
0.6
0.5
Gaseous products included methane, ethane, and oxygen. No carbon monoxide or carbon dioxide was detected. Methane gradually increased in yield from 24.2 mol % at 130 "C to 44.3 mol % at 170 "C. The methyl radical from step h, was probably one of the most reactive radicals in the system. Hydrogen abstraction reactions would be the most probable reaction path for this radical. Either the solvent or in this case the unreacted CHP would serve as a viable reactant. Other termination steps could involve dimerization reactions, steps m and n. A small quantity of ethane, 0.2-0.3 mol % ,was observed at alltemperatures. The methane-to-ethane ratio shows that hydrogen abstraction was greatly favored over dimerization, 148:l at 170 "C. Ethane was the longest chain alkane observed in any of the runs at 130-170 "C. Oxygen was observed in all of the runs and ranged in concentration from 4.1 mol % at 130 "C gradually increasing to 6.7 mol % at 170 "C. Of the radicals generated by the processes depicted by steps g-n, the cumylperoxyradical generated from steps i-k was probably the least reactive radical and consequently the radical in highest concentration in the system. It would thus be selectively expected to form a major quantity of the observed termination products. The most likely source of oxygen, step 1, could result from such a termination step involving the peroxy radical. It was not surprising that oxygen was observed in a relatively low yield. In complex chemical systems employing codopants, i.e., aldehydes along with a hydroperoxide, oxygen has not been observed.21 In a reaction matrix involving fuels and hydroperoxides,no molecular oxygen was observed. This undoubtedly was based on the reaction of other very reactive species such as olefins and heteroatomic compounds that are present in a complicated chemical matrix such as fuels. Dicumene peroxide was a termination product observed in all runs. It was most likely the result of a process involving the cumyloxy radical, step m. The yield of this termination product was observed to gradually increase with temperature from a yield of 2.5 mol 7% at 130 "C to 4.1 mol % at 170 "C. For minor products Scheme 2, steps n-r, is proposed. In radical systems, detailing minor products was difficult. These reactions represent reasonable steps. Another minor product observed was the three-membered cyclic ether a-methylphenyloxirane. It was observed at all temperatures and probably was formed by a rearrangement reaction of the CHp.8~23
Under acidic reaction conditions, the ionic mechanism step p would be the first step in a process culminating in both acetone and phenol, steps p r . Acetone and phenol are both present at small but relatively constant amounts, gradually increasing with temperature. The solvent and the CHP were purified to be free of acidic contamination. However, with reactions involving hydroperoxides trace products observed included carboxylicacids. Of the many pathways by which acidic species could form, the most probable would be the reaction of alcoholsand other active oxygen species that were present. About 1% of the GC/ MS total peak area could not be identified other than the peaks were associated partially oxidized compounds. Undoubtedly acidic species could be present. Thus, an ionic mechanism to account for these products, depicted in step r, was borne out by the similar yields of these two trace products. Consequently, based on product analysis, the thermal cleavage of CHP itself into ions does not occur to any significant extent in benzene solvent. GC/MS analysis indicated several trace products that were not quantitated. They were not always observed in duplicate runsand were always in low concentration,0.0010.01 mol 7%. Products included partially oxidized compounds, dicumene, and a-methyl styrene. Gravimetric storage instability test results are given in Table 2. When the fuel, D-11, alone was thermally stressed, little degradation was observed (0.1 mg/100 mL of fuel). The presenceof the antioxidant AO-30exerted a stabilizing influence on sediment formation in addition to holding down the observed hydroperoxide concentration. However, when the CHP dopant was added, the results in Table 2 showed that the antioxidant AO-30 was overwhelmed. Table 2 also showed that the sedimentation yield increased with increasing hydroperoxide concentration. The yield of insolubles ranged from 0.1 mg in the fuel itself to 0.9 mg with the low CHP concentration to 1.8 mg with the high CHP level. More importantly, the empirical formulas of the sediments in the absence and presence of added CHP (highCHP concentration) were practically identical. Analysis of the sediments by GC/MS and IR showed no incorporation of the cumyloxy radical or cumenol. GC/MS analysis of the stressed fuel filtrates showed the presence of the major decomposition products of CHP, acetophenone and cumenol. The minor and trace products were masked by the fuel components. Since the stress protocol was for vented bottles, no gaseous products were observed. Thus, the CHP is serving as a radical initiator for the sedimentation process rather than a direct participant in the reaction itself. Conclusions
Hydroperoxides are reactive species which are known to be present in turbine fuels. The observed deterioration in fuels can manifest itself in many ways, including the formation of insoluble sediments and gums both in storage and in the combustion chamber. Trace quantities of compounds such as carboxylic acids have been implicated in sediment formation. This paper specificallyexamined cumene hydroperoxide decomposition in deaerated ben (23) Hiatt, R.; Mill, T.; Irwin,K. C.; Castleman, J. K. J. Org. Chem.
1968,33, 1421.
Decomposition of Cumene Hydroperoxide
Energy & Fuels, Vol. 8, No. 4,1994 866
Table 2. Insolubles (mg/100 mL of Fuel) and Peroxide Numbers (ppm of Oxygen) for Fuel D-11 with and without Added Cumene Hydroperoxide for the 80 O W 1 4 days Stress Matrix. sediment insolubles for D-11 insolubles low CHP insolubles for high CHP empirical Fuel D-11 peroxide no. for low CHP peroxide no. high CHP peroxide no. formula none CO.1 3.7 none Ci4.nHis.dNS)iOi.s 0.1 3.7 0.9 141.6 1.8 509.4 Cir..rHu.e(NS)iOi.s 0 CHP concentration: low = 3.2 X 1W M and high = 9.6 X 1b2 M.
zene solvent in the temperature range 130-170 "C. This heat load is one that a typical turbine fuel might be exposed to. Cumene hydroperoxide represents a good model compound for the type of peroxide species present in a middle distillate fuel. A common variety of products was observed at all temperatures. The yield of individual components, however, varied significantly with temperature. The 170"Ctemperature produced the greatest yield of products. Major reaction products included acetophenone, cumenol, methane, oxygen, and dicumene peroxide. Minor reaction products included acetone, ethane, a-methylphenyloxirane, and phenol.
Product analysis for the model system indicated that a free radical mechanism was the predominate reaction pathway. A secondary reaction pathway that was catalyzed by trace acid components accounted for the formation of the minor products. No evidence of benzene solvent participation was noted in any of the experimental runs. Gravimetric analysis of the sediments formed in instability tests reveal that the empirical formula of the sediment formed by the fuel was practically identical to the sediment formed by the addition of the CHP dopant. This indicated that the CHP was serving as a free radical initiator rather than as direct participant in the sedimentation process itself.
